If a rubber band is stretched a particular distance, the restoring force is defined as the force needed to
restore the rubber band to its original form. When a
particular rubber band is stretched to a length of 30
centimeters, the restoring force is –35 newtons, and
when it is stretched to a length of 90 centimeters, the
restoring force is –125 newtons. For every additional
20 centimeters from its original form, the restoring
force decreases by c newtons, where c is a constant.
What is the value of c ?

The dean of students mailed a survey to a total of 400 students. The sample included 100 students randomly selected from each of the freshman, sophomore, junior, and senior classes on campus last term. What sampling method was used?

Solution: The sampling method used here is stratified sampling. Because the sample included the students from each of the freshman, sophomore, junior and senior classes.

Stratified sampling is is a probability sampling method and a form of random sampling in which the population is divided into two or more groups (strata) according to one or more common attributes and and a sample is selected by some design within each group.